<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>CSS Masking: mask-size: mask layer size</title>
    <link rel="author" title="Astley Chen" href="mailto:aschen@mozilla.com">
    <link rel="author" title="Mozilla" href="https://www.mozilla.org">
    <style type="text/css">
      #outer {
        border: 1px solid black;
        width: 64px;
        height: 128px;
      }

      #inner {
        border: 20px solid white;
        width: 24px;
        height: 88px;
      }

      #innermost {
        width: 24px;
        height: 24px;
        background-color: purple;
      }
    </style>
  </head>
  <body>
    <div id="outer">
      <div id="inner">
        <div id="innermost"></div>
      </div>
    </div>
  </body>
</html>
